(a) Field of the Invention
The present invention relates, in general, to the formation of containers having a thickened flange. More specifically, the present invention discloses a process and apparatus for manufacturing blow-molded containers having thickened flanges. Containers produced by the inventive process and apparatus are also disclosed.
(b) Description of Related Art
Typically, blow-molding processes call for suspension of a heated tube or parison of thermoplastic material between two mold halves. The mold halves are closed on the suspended parison, then an expanding medium (e.g., air) at super atmospheric pressure is introduced into the tube. Alternatively, the mold may be evacuated, i.e., via vacuum, or both blow-molding pressure and vacuum may be combined so that the material of the tube eventually conforms to the contour of the mold. The mold is generally much cooler than the thermoplastic material, and hence, the thermoplastic material becomes rigid or "sets" after being in contact with the mold for a short period of time. Once the thermoplastic material sets, the mold halves are opened and the part is removed.
Owing to the particular qualities of previously-known blow-molding process, the open top containers produced thereby have not possessed a very rigid rim or top section. This has been particularly true with those containers with continually outwardly and upwardly tapering side walls wherein the top or rim constitutes the largest part of the container. In order to augment the strength of the top rim, conventional containers have been formed having an annular projection at the top thereof; the projection having a generally channel-shaped or multiple channel shaped cross section. This channel shaped cross section, although beneficial, often does not provide the rigidity required in many applications. Furthermore, the channel shaped section is located at the largest radial dimension of the mold where parison stretch is the greatest. For example, U.S. Pat. No. 4,972,963 to Guarriello et al. discloses a blow-molded article with a reverse lip. As shown in FIG. 3, the "reversed lip" refers to a generally U-shaped member which, by its shape, provides somewhat increased strength to the top of the container. The reversed lip has the same thickness as the rest of the container, however, i.e., one layer of blow-molded material.
In order to provide flanges having strength beyond the limits possible with a single thickness of blow-molded material, a variety of methods for producing thickened flanges have been developed. For example, in U.S. Pat. No. 4,713,207 to Udell et al. a process for producing blow-molded containers (e.g., drums) having thickened flanges and/or chimes is disclosed. The process calls for a ring of extrudate (E) to be placed in the mold before the parison (P) is extruded into the mold chamber. In this way, as the parison is blow molded, the extrudate (E) and parison (P) materials may bond together.
Placing two different materials within the mold chamber is time consuming and expensive, however. Therefore, other processes have been developed which form a thickened flange from a single parison of material. For example, U.S. Pat. No. 4,117,062 to Uhlig discloses a process for producing containers (e.g., drums) having thickened chimes and/or flanges. In this process, the mold chamber is configured to keep certain parts of the blow-molded container warmer than others (i.e., those sections which will be refolded). After the parison of thermoplastic material has been expanded, the desired portions of material are refolded to form flanges/chimes by axially moving the ring (26) (see, FIGS. 3-7). It does not appear that the layers of blow-molded material bond together after being folded over, however, and the simultaneous formation of two containers is not disclosed.
In other processes, the folded over layers of blow-molded material are substantially fused together, thereby increasing the strength of the flange. For example, in U.S. Pat. No. 3,843,005 to Uhlig (which is hereby incorporated by reference), the folded over layers may be bonded together by residual heat (see, e.g., column 11, lines 33-37). In this process (as is common in the prior art), however, the thickened flange (e.g., bottom support wall (99) in FIG. 3) is formed by moving one major mold section relative to another (e.g., plates (84) in FIGS. 15-17) in an axial direction, thereby moving a substantial portion of the mold and container. Since the molds are often quite heavy, this movement can be energy-intensive and wasteful. Furthermore, as shown in FIGS. 15-17, moving the major mold components often severs the "tail" (85) of thermoplastic material pinched between the mold sections. Severing the tail (85) is detrimental since this makes it more difficult to handle the container after it is released from the mold without marring the container.
Other processes have been developed wherein thickened flanges are formed by the movement of major mold sections. For example, U.S. Pat. No. 4,177,934 to Hammes et al. (which is hereby incorporated by reference) discloses a process for producing containers having a thickened flange wherein the folded over blow-molded material bonds to itself to form an integral flange (3). The flange (3) is formed by the axial movement of mold elements (10 and 11).
Finally, U.S. Pat. No. 5,026,268 to Lee discloses an apparatus for forming blow-molded containers having thickened flanges by folding over the thermoplastic material. The blow-molded material is folded over by the relative motion of three longitudinally-arranged major mold sections (36, 38, 40).
While these methods are capable of producing very useful containers, the problem of producing blow-molded containers having thickened flanges, the layers of which reliably bond together, without relying upon movement of major mold sections has remained. Furthermore the corollary problem of post-release (i.e., from the mold chamber) handling of the container also remains unsolved, i.e., in light of the tendency of present processes to remove the "tail" of extruded thermoplastic material pinched between the mold sections.